I Too have looked into tracfone and may also be buying one when we go over.
As to your questions Sweeney Todd
If you we to call you brother the call is sent to the UK (This is the part of the call you pay for) and then it is sent to your brother in the US even if he is standing next to you (And he will pay for receiving it) This is around 75p to £1 depending on his network.
So this will be almost as expensive as you both using UK contract phones.
The only way Tracfones are any cheaper is if you both have them or to call US landlines.
Question 2
Sorry do not know the answer to that one.
If you do not keep your tracfone activated you WILL loose the number. All you need to do is next time in the US is phone tracfone and they will give you a new number and airtime.
Hope this helps.